This invention relates to an optical module suitably used in an optical communication apparatus, sensor or the like, and to a fiber stub type optical device mountable in an optical module and having a built-in optoisolator for blocking reflected and returned light from the outside, and to a method for producing a fiber stub type optical device.
In a semiconductor laser diode (hereinafter, xe2x80x9cLDxe2x80x9d), which is one of semiconductor devices, used as a light source in the optical communication, if emitted light is reflected and returned to an active layer of the LD, an oscillating state of the LD is disturbed. This causes a variation in emission power and a wavelength deviation, thereby deteriorating signals.
In order to prevent such a problem, the LD is normally mounted in the same package as an optoisolator for transmitting light only in one direction to thereby construct an LD module which is one type of the optical module.
Particularly, analog signals are likely to be deteriorated by the reflected and returned light, and the higher the density thereof, the more they are likely to be influenced by the reflected and returned light. Accordingly, optoisolators have become essential elements as analog transmission data via, e.g., CATV increases and requires larger capacity and higher speed.
The operation of a general optoisolator is described. The optoisolator is, as shown in FIGS. 26A and 26B, comprised of a Faraday rotator 19c and two polarizers 19a, 19b at the opposite sides of the Faraday rotator 19c. As shown in FIG. 26A, a forward propagating light 22 incident on the first polarizer 19a becomes a linearly polarized light of a specific polarization direction (see 23a in FIG. 26A). This forward propagating light 22 has its polarization direction 24 rotated by 45xc2x0 to the right with respect to the propagating direction of the light in FIG. 26A by the Faraday rotator 19c, is then incident on the second polarizer 19b having the polarization direction 23b rotated by 45xc2x0 to the right from the polarization direction 23a of the first polarizer 19a with respect to the propagating direction of the light, and emerges out of the optoisolator while maintaining its polarization direction.
On the other hand, as shown in FIG. 26B, backward propagating light 25 is made into a linearly polarized light by the second polarizer 19b, and has its polarization direction 24 rotated by 45xc2x0 in the same direction as in FIG. 26A by the Faraday rotator 19c, with the result that this light is blocked by the first polarizer 19a by forming an angle of 90xc2x0 with respect to the polarization direction 23a of the first polarizer 19a. 
Next, an example of the conventional LD module is described. As shown in FIG. 27, an LD module J1 is constructed such that at least an LD 15, lenses 6a, 6b, an optoisolator 2, and one end of a single-mode fiber 4 are accommodated in a package 18. Identified by 16 in FIG. 27 is a light detector, by 17 a Peltier cooler and by 32 a rubber boot for protecting an optical fiber margin.
In this LD module J1, the light emitted from the LD 15 is collimated by the lens 6a, transmits through the optoisolator 2, and is gathered by the lens 6b to be incident on the single-mode fiber 4. The package is used to isolate the respective parts from external environments. Ball lenses, biconvex lenses, aspheric lenses, gradient-index lenses (hereinafter, xe2x80x9cGRIN lensesxe2x80x9d) and the like are used as the lenses 6a, 6b. 
Further, in order to miniaturize the entire module and facilitate an alignment, there has been proposed an optical device J2 which is a combination of an optical fiber and an optoisolator without using lenses as shown in FIG. 28 (see Japanese Unexamined Patent Publication No. 9-105886). In this optical device J2, a core enlarged fiber 4 obtained by enlarging a core diameter of an optical fiber is used, the elements (polarizers 19a, 19b, Faraday rotator 19c) of the optoisolator are separately arranged and while a specified element (Faraday rotator 19c) is arranged in a groove 7 formed obliquely to an optic axis in order to prevent reflection.
In order to miniaturize the entire optical module and facilitate an alignment, there has been also proposed an optical device J3 in which an optoisolator is mounted on a fiber stub similarly using a core enlarged fiber 5 without using lenses as shown in FIG. 29 (see Japanese Unexamined Patent Publication No. 10-68909).
This optical device J3 uses the core enlarged fiber 5 obtained by enlarging a core diameter of an optical fiber in order to improve optical coupling, and the optoisolator 2 is obliquely inserted with respect to an optic axis in order to prevent reflection. The optical device J3 is constructed by fitting an optoisolator 2 and a cylindrical magnet 30 surrounding the optoisolator 2 in a ferrule 3 holding a fiber 9 having a spherical end in its axial center and fixedly mounting an entire assembly in a sleeve 13.
The optical device J3 is free from radial displacement since the ferrule 3 is coaxially mounted with high precision. Further, the module provided with the optoisolator can be easily assembled by operation steps similar to those for a module having no optoisolator. However, in this prior art, how the ferrule 3 is processed, how the optoisolator 2 and the magnet 30 are assembled and fixed are unclear.
The core enlarged fiber used in the optical devices J2, J3 and the like is formed by locally heating a general single-mode fiber. Specifically, the single-mode fiber is heated to diffuse dopants such as germanium in the core, thereby enlarging a diffusion area of the dopants and making a specific refractive index difference smaller.
If a diameter of the core increases with the specific refractive index difference between the core of the optical fiber and the cladding thereof unchanged, a single-mode condition breaks and a multimode is excited. However, in the case of the core enlarged fiber, the enlargement of the core and a reduction of the specific refractive index difference simultaneously occur due to the diffusion of the dopants caused by heat and accordingly rxc3x97(D)xc2xd is automatically maintained at constant value. Here, r denotes a radius of the core of the optical fiber, D a specific refractive index difference between the core and the cladding, and rxc3x97Dxc2xd an amount in proportion to normalized frequency. The single-mode condition is maintained if this value rxc3x97(D)xc2xd is constant.
FIG. 30 shows optical coupling characteristics of the core enlarged fiber. A horizontal axis represents a fiber spacing which is a spacing between the divided sections of the core enlarged fiber (width of a groove formed in a core enlarged portion) and a vertical axis represents an optical coupling loss (diffraction loss). Here, w denotes a mode field diameter and corresponds to the respective curves. It is assumed that wavelength is 1.31 xcexcm generally used in optical communication and the groove (clearance between fibers) is filled with air (refractive index n=1).
This graph shows that the larger the mode field diameter w, the smaller the diffraction loss. For example, in the case of the mode field diameter w of 10 xcexcm (i.e., the core is not enlarged), when the fiber spacing is 70 xcexcm, the diffraction loss is above 1 dB. Contrary to this, in the case of the mode field diameter w of 40 xcexcm, even when the fiber spacing is 800 xcexcm, the diffraction loss is below 1 dB. This clearly shows an improvement in coupling characteristic.
However, in the module as shown in FIG. 27, the optoisolator 2, the lenses 6a, 6b and other parts are aligned after being separately fixedly mounted in the holder. Thus, this module disadvantageously requires many parts and a cumbersome adjustment and results in a large size.
Although the core enlarged fibers are used in the examples shown in FIGS. 28 and 29, the conventional core enlarged fibers have following problems (a) to (g).
(a) Although having an advantage of an alleviated tolerance for radial displacement, the core enlarged fiber requires a tapered portion where the core diameter gradually increases. A large temperature difference is required to form the tapered portion and, thus, heating must be locally applied.
(b) Since the core enlarged portion is several mm, this heating needs to be applied to each device at least once. In the case of heating by an electric furnace, several to in the order of ten hours are needed. That the core enlarged fiber has to be formed by locally applying heating one by one for a long time is a biggest problem thereof.
(c) In the case that the grooves for inserting the optical devices are formed, loss is large since, if the tapered portion is located in the groove, this results in coupling of fibers having different mode field diameters.
(d) The length and angle of the tapered portion are determined by a temperature gradient which is difficult to control. Loss changes as the length and angle change.
(e) In the optical device J3 shown in FIG. 29, the core enlarged fiber 5 needs to have a tapered portion and a long core enlarged region. It is actually difficult to manufacture such a core enlarged fiber because the tapered portion which requires a sharp temperature gradient and the long core enlarged region which requires a uniform temperature are adjacent to each other. Specifically, it is very difficult to controllably form the core enlarged fiber having the tapered portion with the parallel core enlarged region of a desired length.
(f) In the optical device J3 shown in FIG. 29, the groove dividing the ferrule 3 as well as the core enlarged fiber 5 is formed to arrange the optoisolator 2. This has an advantage that the axis of the core enlarged fiber 5 is not displaced even after being divided. However, since the ferrule 3 is usually made of a ceramic such as alumina or zirconia having excellent wear resistance, strength and precision, its hardness and elasticity are considerably different from those of the core enlarged fiber 5. If a blade for cutting the ceramic is used, a cut face of the core enlarged fiber 5 is rough, which results in diffusion of light and, therefore, a larger loss.
(g) In the optical device J3 shown in FIG. 29, since the optoisolator 2 is obliquely arranged, even light slightly reflected is at an angle to the optic axis of the core enlarged fiber 5 and the optoisolator 2 is coupled to the core of the optical fiber to prevent transmission of light in backward direction. However, a part of the reflected light may be incident on the cladding and transmits therethrough in a direction toward the LD (cladding mode).
It is an object of the present invention to provide a fiber stub type optical device, an optical module provided with a fiber stub type optical device, and a method for producing a fiber stub type optical device which are respectively free from the problems residing in the prior art.
According to an aspect of the invention, a fiber stub type optical device comprises a ferrule formed with a through hole extending in a longitudinal direction thereof and a groove for dividing the through hole in an intermediate position with respect to the longitudinal direction; a first optical fiber accommodated in the through hole; a second optical fiber aligned with the first optical fiber in the through hole while being divided by the groove and having a larger mode field diameter than the first optical fiber; and an optoisolator provided in the groove and optically connected with the second optical fiber divided by the groove.
With this construction, the fiber stub type optical device can be assembled substantially alignment-free even if an optical element is inserted in a transmission path, and by a far simpler procedure for a shorter time than the prior art.
Another aspect of the invention is directed to an optical module, comprising a substrate; and a fiber stub type optical device provided on the substrate and comprising a ferrule formed with a through hole extending in a longitudinal direction thereof and a groove for dividing the through hole in an intermediate position with respect to the longitudinal direction; a first optical fiber accommodated in the through hole; a second optical fiber aligned with the first optical fiber in the through hole while being divided by the groove and having a larger mode field diameter than the first optical fiber; and an optoisolator provided in the groove and optically connected with sections of the second optical fiber divided by the groove.
With this construction, the optical module can be optically adjusted only by adjusting the positions of the fiber stub type optical device and an optical element such as a light emitter.
Thus, the optical module can be assembled by a far simpler procedure for a shorter time than the prior art.
Still another aspect of the invention is directed to a method for manufacturing a fiber stub type optical device, comprising the steps of preparing a ferrule formed with a through hole extending in a longitudinal direction thereof; aligning a first optical fiber and a second optical fiber having a larger mode field diameter than the first optical fiber in the through hole; forming a groove in an intermediate position of the ferrule so as to divide the second optical fiber accommodate in the through hole and cross the through hole; and providing an optoisolator in the groove so as to be optically connected with sections of the second optical fiber divided by the groove.
Further aspect of the invention is directed to a method for manufacturing a fiber stub type optical device, comprising the steps of preparing a ferrule formed with a through hole extending in a longitudinal direction thereof; forming a groove in an intermediate position of the ferrule; aligning a first optical fiber and a second optical fiber having a larger mode field diameter than the first optical fiber in the through hole, such that the second optical fiber extends over the groove; dividing the second optical fiber accommodated to extend over the groove; and providing an optoisolator between sections of the second optical fiber divided by the groove.
According to the above methods, the fiber stub type optical device can be assembled substantially alignment-free even if an optical element is inserted in a transmission path, and by a far simpler procedure for a shorter time than the prior art.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.